1. Field of the Invention
The present invention relates to a sealing structure for an electric motor, and more particularly, it pertains to a fluid seal which is disposed in a radial gap between a shaft and a hub of a motor for preventing contaminates such as oil, dusts and the like within the motor from leaking to the outside of the motor. In addition, the present invention relates to a spindle motor having such a fluid seal, and a disk drive using such a spindle motor.
2. Description of the Related Art
FIG. 16 schematically illustrates the internal construction of a typical disk drive 70. The disk drive 70 comprises a housing 71 having a motor 72 mounted therein for driving a disk 73 capable of storing various kinds of information in digital format and at high density. The housing 71 also contains a head assembly 77 for reading and writing information to or from the disk plate 73. The head assembly 77 comprises a head 76 for reading and writing information on the disk 73, an arm 75 for supporting the head 76, and an actuator portion 74 for moving the head 76 and arm 75 to a required position. In recent years, the information density capable of being stored on a disk has grown by leaps and bounds. A clean environment for installation and housing of a disk containing extremely little dust is required. Therefore, the inside of the housing 71 of the disk drive 70 has to form an extremely clean space and to this end is shut off from outside air. Also, the motor 72 used in a disk drive may not leak contaminates, such as oil mists and the like. In response to such a demand, fluid seals are used to prevent oil mists and the like that are caused by rotation of the shaft and ball bearing portions of the motor 72.
In FIG. 15, an example of spindle motor with a fluid seal is shown as a cross-sectional view cut away by a plane passing through the shaft axis of the motor. In particular, shaft 1 is fixed and has a rotor hub 4 rotatably disposed thereon by upper and lower ball bearings 2 and 2xe2x80x2. A stator core 10 and a coil 9 are disposed on a bracket 7 which constitutes a stationary portion, that is, the rotor hub 4 has the shape of a so-called outer rotor. A seal portion 3 is situated above the upper ball bearing, and is fitted in between the shaft 1 and the rotor hub 4 so as to fill the gap therebetween.
Hitherto, a magnetic fluid in schematic in FIG. 13 has been used to form the seal portion 3 FIG. 15. Since the magnetic fluid seal has a rotationally symmetrical shape with respect to the alternate long and short dash line exhibiting the shaft axis in the figure, only the right sides of the cross-sectional views cut away by a plane passing through the shaft axis is shown in the Figures below.
The magnetic fluid seal shown in FIG. 13 comprises annular pole pieces 30 and 33 formed by of a pair of magnetic bodies, and an annular magnet 32 sandwiched therebetween. These pole pieces and magnet are disposed above the outer ring portion 21 of the upper bearing 2 via a holder 35, and are fixed by an adhesive 34. The inner peripheral end portions of the pole pieces 30 and 33 confront the outer sealing surface of the shaft with a gap located therebetween. Between the inner end portion of the upper pole piece and the outer surface of the shaft, a magnetic fluid 31 is held by the flux generated by the annular magnet 32, and this forms an air barrier, which prevents dust generated by the rotation of the lower pole or the like from leaking upwardly along the shaft via the above-mentioned gap.
FIG. 14 is an enlarged view of only the fluid seal portion of FIG. 13. Usually, the magnetic fluid 31 is stored between the upper and lower pole pieces and also in the annular concave portion formed on the inner peripheral side of the annular magnet 32. At a standstill, the magnetic fluid 31 is held forming the meniscus indicated by the broken line in FIG. 14.
In recent years, with the increase in the processing speed of personal computers and the like, storage devices used in them, such as magnetic disks, are increasing in storage capacity and the writing/reading speed for storing information is rising by leaps and bounds. In response to such a situation, the magnetic disk or the like is required to have a rotational speed, of for example, more than ten thousand rotations per minute. In such a case, the magnetic fluid held between the inner peripheral end portion of the upper pole piece 30 and the outer peripheral surface of the shaft is attracted by a rotational centrifugal force in the radial outward direction, and forms a meniscus, as indicated for example, by the solid line in FIG. 14. That is, when the spindle motor is at a standstill, the magnetic fluid 31 forms the meniscus indicated by the broken line, but when attracted by the rotational centrifugal force, for example, a part of the upper portion of the meniscus 311 flows up to the magnetic fluid part 312 on the inner end portion of the upper pole piece 30, while a part of the lower portion of the meniscus 313 is pushed back to the part 314 within the annular concave portion on the inner side of the upper and lower pole pieces. As a result, the sealing layer, formed by the magnetic fluid, held between the inner end portion of the upper pole piece 30 and the outer surface of the shaft becomes thin, and the sealing capability, especially the barrier pressure of the seal decreases. In addition, this location is when the barrier pressure of the seal is at the maximum equivalent pressure required to prevent oil mists and the like from flowing out through the fluid seal.
A magnetic fluid seal shown in FIG. 13 has a thickness in the axial direction of several millimeters (e.g., 5 mm or 3 mm), that is, this magnetic fluid seal is a very thin precision component, although it is a complicated component comprising upper and lower pole pieces 30 and 33, an annular magnet 32, a holder 35, and so on. Manufacturing thereof, therefore, requires numerous precision processes, a large number of man-hours, and a high cost.
Furthermore, in response to the recent reduction in thickness of note-book personal computers and the downsizing or weight-reduction thereof intended for an improvement in portability, the downsizing tendency of storage devices, such as magnetic disks and the like, is being increasingly promoted, and a corresponding decrease in thickness of the motors used in these storage devices also is being desired. With respect to the fluid seals, it is therefore also an essential condition to make these thinner than conventional ones.
Accordingly, it is an object of the present invention to provide a low-profile fluid seal capable of stably maintaining a sealing capability with respect to a high-speed rotation.
It is another object of the present invention to provide a fluid seal having a simple structure and being capable of meeting a high-speed rotation.
It is still another object of the present invention to provide a fluid seal capable of being manufactured with a small number of man-hours and at a low cost.
It is a further object of the present invention to provide a low-profile and low-cost spindle motor capable of meeting a high-speed rotation, and to provide a high-speed rotating disk drive using this motor.
In order to achieve these objects, a fluid seal in accordance with the present invention comprises an annular body having an annular sealing surface confronting the outer sealing surface of a motor shaft with a minute gap therebetween, and a fluid held in this minute gap. In this way, a fluid seal is constructed which has a much simpler structure than that of a conventional magnetic fluid seal.
In addition, in a fluid seal in accordance with a particular embodiment the present invention, the annular sealing surface of the annular body has an approximately concave shape in cross-section. In other words, the nearer to the upper or lower surface of the annular body in an axial direction it may go, the shorter are the distances from the annular sealing surface of the annular body to the axis of the shaft. One advantage of this structure is that it prevents the sealing fluid from being scattered by rotational centrifugal force, and achieves a stable sealing capability even under conditions of high-speed rotation.
In a fluid seal in accordance with another embodiment of the present invention, the outer sealing surface of the shaft has an annular approximately convex shape in cross-section. The annular convex part has an annular tip that is located at a central portion thereof. The annular tip makes contact with the fluid held in a cavity of the approximately concave portion formed on the annular sealing surface of the annular body of the fluid seal. This provides a fluid-sealing structure that maintains a fluid-sealing capability even with a small quantity of fluid.
Also, a fluid seal in accordance with another embodiment of the present invention has an angled sealing structure in which the cross-sectional shape thereof is opened in opposed directions, or upwardly and downwardly. This permits the fluid held in this angled sealing gap to be stably held without being scattered, even when subject to external shocks.
The fluid sealing structure described above can also be applied to a sealing portion provided between an inner ring and an outer ring of a ball bearing (a fluid seal is generally fixed to the inner surface of the outer ring). Specifically, a fluid seal in accordance with another embodiment of the present invention comprises an annular body having an inner surface confronting the outer surface of the inner ring with a minute gap therebetween, and a fluid held in this minute gap. In addition, in a fluid seal in accordance with another embodiment of the present invention, the inner surface of the annular body constituting a fluid seal has an approximately concave shape opened to the axis of the shaft in cross-section when it is cut away by a plane passing through the axis of the shaft. In a fluid seal in accordance with a further embodiment of the present invention, the outer sealing surface of the inner ring has an annular approximately convex part. The annular approximately convex shape has an annular tip which is located at the central portion in the axial direction of the part confronting the above-mentioned approximately concave shape. Also, a further embodiment includes an angled sealing structure in which the cross-sectional shape thereof is opened in opposing directions, or upwardly and downwardly, in the axial direction between the annular sealing surface of the annular body and the outer sealing surface of the inner ring. By these structures, a ball bearing with a seal capable of high stability can be attained even under high-speed rotational conditions.
Furthermore, in accordance with the present invention, a spindle motor is realized which has the above-described fluid seal or ball bearing with the fluid seal. Accordingly, there is provided a low-profile and low-cost spindle motor capable of stably achieving a sealing capability under high-speed rotational conditions. Moreover, in still a further embodiment of the present invention, a disk drive using this spindle motor is realized. Accordingly, there is provided a disk drive having a high data transfer speed as a result of the high-speed rotation.
The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.